High-temperature fuel cells are currently among the most promising sources of electrical power both in mobile systems and stationary high-power electric stations. Additionally, HTFCs are being considered as an alternative to nuclear power.
One important feature of HTFCs is the ability for the direct conversion of the chemical energy stored in several fuel types into electric energy. Due to this direct conversion, the HTFC cycle does not fall under Carnot cycle limitations, and thus it is theoretically possible to achieve a cycle efficiency of up to 80%. Currently, some experimental specimens have achieved an efficiency of 50%, and efficiency values of up to 65-70% are anticipated in the near future. Moreover, when compared with conventional methods for generating electricity, fuel cells have a number of other advantages including: design modularity, high efficiency under partial electrical load, the possibility for the combined generation of electric and thermal energy, several orders lower contaminant product output than current widely used energy sources, and an absence of moving parts and units.
Recently, much research and development has been under way in the area of the internationally classified high-temperature solid oxide fuel cells (SOFC). SOFCs have a number of apparent advantages over other fuel cell types. These advantages may include: usage of cheap oxide materials for the electrodes, absence of liquid circulation i.e. a solid electrolyte, and the absence of liquids within the fuel cells. The utilization of solid oxide electrolyte in ceramic fuel cells eliminates the need for monitoring the electrolyte and excludes material corrosion problems normally incurred from the use of a liquid electrolyte. Conventional ceramic fuel cells operate at high temperatures (over 600° C.). It is desirable for fuel cells to operate at high working temperatures, the elevated temperature increases the reaction rate, allowing the cell to convert a hydrocarbon fuel within the cell into energy (internal reforming) and to generate high-potential heat suitable for regeneration and utilization in the main cycle. Thus, power plants based on certain ceramic fuel cells can be simple and more effective than many other known techniques of producing electrical and thermal energy. Moreover, since all components of the HTFCs are in a solid state, the ceramic fuel cells, for example, can be formed into ultra-thin layers, and the cell's elements can be shaped into unique forms unachievable in liquid electrolyte fuel cell systems.
On the other hand, the ceramic fuel cells place increased requirements on the materials and techniques used in manufacturing their components. Production of ceramic powders and the development of methods for forming ceramic powders play key roles in the technology of ceramic fuel cells.
The main components of the ceramic fuel cell are the electrolyte, the anode, the cathode, and the current passage. Within the fuel cell each component performs several functions and has to meet certain requirements including: feature stability (chemical, phase, structural and dimensional) in oxidizing and/or restorative media, a chemical compatibility with other components, and a proper conductivity. Additionally, the components of ceramic fuel cells must have similar thermal expansion coefficients in order to eliminate peeling and destruction during the manufacturing process and in operation. The electrolyte and the current passage must be sufficiently dense in order to prevent the mixing of gases in the anode and cathode spaces, while the anode and the cathode must be porous enough to provide gas transport to the reaction location and to facilitate the removal of the reaction products.
In addition to the above mentioned requirements, the cell components must possess a high strength and resistivity while enabling the possibility for a simple and cheap method for cell manufacture. Moreover, methods for manufacturing the components of ceramic fuel cells must be compatible, because cell manufacturing conditions cannot be divided and independent for each component. For example, if the components are manufactured and joined one by one, then the caking temperature of each subsequent component must be equal to or lower that of the caking temperature of the previous component, in order to avoid a change in the microstructure of the previous component. If the components are formed in raw form, then all components must be caked at the same temperature modes. Moreover, the components of the ceramic fuel cell must be compatible not only at operational temperatures but also at higher temperatures at which the forming of ceramic structures takes place.
The current technologies for manufacturing HTFCs and their components which are being widely used, in general, meet oil tile chemical stability, thermal resistivity, electrical and other features requirements. Component compositions are mainly the ceramic materials based on zirconium dioxide, oxides of cerium, thorium, barium, strontium, bismuth, compounds of perovskite type materials based on oxides of chromium, manganese, cobalt, nickel, and lanthanum modified by magnesium, calcium, strontium, barium, scandium, yttrium, cerium and other lanthanides. In the technology of manufacturing the materials for the HTFCs, all known methods for manufacturing ceramic materials are applicable. However, increasing and complicated specific demands made on HTFC construction, such as:
the predetermined porosity of the ceramic electrodes with sufficient structural strength and electrical conductivity;
and decreasing the thickness of the electrolyte film while maintaining the gas density and as a consequence; and
the necessity to form thin electrolyte films on the porous carrier electrodes with a maximum increase of a specific working surface per 111-FC weight unit;
substantially limit the application of known ceramic technologies and materials when forming HTFC components such as the electrodes, the electrolytes, and the current passages.
One of the limitations is conditioned by great differences in caking temperatures of the materials, at which the HTFC components being mated are manufactured. For example, the caking of the 10YSZ electrolyte takes place at a temperature of 1700° C., while the carrier cathode of lanthanum-strontium-La0.7Sr0.3MnO3 is caked at 1450° C. At the same time the specific caking temperatures for every material are necessary to fully stabilize the characteristics used in the HTFCs. Therefore, special methods for forming the solid oxide electrolytes and the electrodes developed recently appear to be inefficient. These methods are based generally on maximizing an increase of the powder's activity in order to decrease the differential between the caking temperatures of electrolytes and electrodes. In reality, these methods succeed in forming the surfaces of the HTFC contacts i.e. cathode/electrolyte/anode, but at temperatures ranging from 100-400° C. lower than the usual ones. However, materials incorporated herein which prove to be sufficiently active, continuously change toward the final phase structure during the article exploitation from 900°-1100° C. This change may be accompanied by irregular shrinkages of various HTFC components, and increased mutual diffusion between components, which may lead to component failure or to unacceptable decreases in desired article features.
Certain ceramic technologies such as isostatic pressuring, extrusion, plasma spraying, vacuum spraying etc. used in forming HTFC components from powders can be modified using additional techniques, such as increasing the electrolyte density, providing the determined porosity, or increasing the adhesion of the elements to be mated, to utilize the active powders either during the process of forming the components itself or at other points in the manufacturing process.
A method for manufacturing the HTFC includes the steps of applying consecutively a fuel electrode layer, an electrolyte layer of YSZ, and an air electrode layer, to make a three layer element on a carrier substrate made of the CSZ is well known in the art. The thermal expansion coefficient of the substrate is matched to the thermal expansion coefficient of the applied electrolyte layer by exposing the substrate to a thermal treatment during the application step. The thermal treatment is performed by heating at the rate of 50° C. per hour until attaining 1450° C., holding at 1450° C. for 6 hours before decreasing at the same rate. (U.S. Pat. No. 5,021,304, Int. Cl. H01M 8/10, H01M 4/86, published 1991).
When manufacturing the HTFC according to U.S. Pat. No. 5,021,304, a wide range of initial substances and compounds may be used. The manufacture of the individual components of an HTFC is performed by various technologies, which finally complicates the process of manufacturing the HTFC as a whole and particularly complicates its hardware realization.
An alternative to the given method of forming HTFC components, certain technologies may use processes based on pyrolysis of metals combined with various organic reagents containing the elements to be incorporated into the components.
By analyzing technical decisions, one may make the conclusion that the role of the organic reagents in manufacturing the electrolytes and the electrodes is similar to that regarding the caking (forming) temperature decrease. However, this role is opposite regarding the desirable final result, since the electrolyte must be gas-dense, and the electrode must be porous. After performing this analysis, a tendency develops to use different classes of organic substances, which ultimately leads to inevitable widening in the range of substances and materials used, and subsequently a rise in the cost of manufacturing the HTFC as a whole.
Thus, if β-diketones are mainly used in the methods for forming the electrolyte, the other various classes of organic substances: alcohols, carbonic acids, amines, and many other (including easily inflammable) ones are used in the methods for forming the electrodes.
In this context, the process for manufacturing the materials for forming any HTFC components must be universal and fit within the limits of the processes and equipment used in manufacturing both the materials themselves and the HTFC components based on these materials, thereby significantly reducing the article cost, the range of used materials and substances, and the amount of technological equipment used.
The ceramic cathode can be the carrier construction base of the HTFC. The following main requirements are placed on the carrier cathode of the fuel cell:
the overall cathode porosity and the cathode pore size must provide a free supply of oxygen-containing gas to the three-phase cathode/electrolyte/gas border;
a sufficient mechanical strength in order to provide reliable long-term operation of the fuel cell;
the thermal expansion coefficient (TEC) of the cathode must be approximately that of the TEC of the solid electrolyte, in order to avoid the occurrence of mechanical stresses leading to a failure in the solid electrolyte layer.
The first two requirements appear to contradict each other, therefore the problem of finding a compromise always exists in practice.
The third requirement is mainly provided for by carefully choosing the correct materials. For example, oxide compounds of La, Mn, Cr, Co, or Ni doped with oxides from the group including Mg, Ca, SY, or Ba are used as a material for manufacturing the cathode.
Considering the manufacturing requirements and the high working temperatures (on the order of 1000° C.), the YSZ-based SOFC comprised of noble metals or oxide compounds can be used for manufacturing the cathodes. However, due to their high cost, the noble metals such as platinum, palladium, and silver are practically used in SOFCs for research purposes only. Most recently, doped lanthanum manganite LaMnO3 became the most widely used material for these purposes.
As it was mentioned above, the choice of the material for manufacturing the porous cathode is performed according to its conductivity, TEC and other features. In so doing, a temperature for the preliminary burning (synthesis) of the selected material is predetermined (i.e., stipulated temperature at which the material obtains a necessary phase composition) and cannot be changed over a wide range. This, in its turn, does not allow the diffusion and strength characteristics of the porous carrier cathode to vary. Moreover, caking at high temperatures assumes higher power expenses for manufacturing the fuel cells.
It is known from the prior art, that methods for manufacturing the electrodes differ depending on the HTFC construction.
In constructions with the carrier electrolyte, the manufacturing of the electrodes (in most cases) consists of applying a thin-dispersed suspension of material in some solvent such as alcohol, acetone etc. onto the electrolyte surface, then burning at a temperature which will provide reliable adhesion of the electrode material to the electrolyte. The application of the electrode mass onto the electrolyte surface can also be performed by painting, dipping or spraying. Other methods of manufacturing the electrode include: chemically condensing the carrier solid electrolyte onto the surface from liquid solutions or gaseous phase materials; thermal decomposition of metal salts; joint hot-compacting the electrode material and the electrolyte; or by spraying [M. V. Perfiliev, A. K. Demin, B. L. Kuzin, A. S. Lipilin, “Hightemperature gas electrolysis”, Moscow, Nauka, 1988, p. 98].
In constructions with the carrier electrode, the electrodes are manufactured from the specially prepared formable masses using various possible forming techniques including: extrusion, single-axis or isostatic compacting, or a casting.
A method for manufacturing the tubular carrier electrode for the solid oxide electrolyte of an electrochemical fuel cell is most similar to the claimed group of inventions in the aspect of manufacturing the cathode by the technical essence and achieved result. The method comprises steps of: dry mixing the powders of MnO2, CaCO3 and La2O3 (in order to obtain the lanthanum manganite (LaMnO3) doped with calcium after the caking); compacting the obtained mixture into briquettes; synthesizing the compacted briquettes at high temperature by means of caking; further grinding the briquettes until obtaining a powder with a predetermined size of particles; mixing the prepared powder with removable items: a plasticizer, a pore creating agent, and a water-soluble substance in order to obtain a formable mass; forming this mass into thin-walled tubes; and finally caking these tubes (U.S. Pat. No. 5,108,850, Int. Cl. H01M 4/88, published in 1992).
Besides limitations inherent to known similar designs, the solid-phase synthesis used in the technology protected by this patent does not provide the homogenous characteristics of the powder. Moreover, it contemplates the usage of the plasticizer, pore-creating agent, and water-soluble substance. The caking process is performed at high temperatures with significant shrinkages, thereby requiring additional techniques to obtain articles of a required size, i.e. the technology is sufficiently labor-consuming and power intensive.
Current high-temperature fuel cells having solid oxide electrolytes can be divided into two classes 1) conduction by oxygen ions and 2) conduction by hydrogen ions (protons). Among the many high-temperature solid electrolytes being researched, two types are considered promising because of their physical and chemical characteristics and chemical composition:
an electrolyte with ionic oxygen conductivity on the basis of a modified zirconium dioxide;
an electrolyte with ionic proton conductivity on the basis of a modified barium cerate (or strontium cerate).
Currently, many firms are working on ways of manufacturing from 1 to 10 k W generators including: Westinghouse Electric Corp. (USA), Fuji Electric Co-(Japan Asea Brown Boveri AG (Germany), NGK Insulators Ltd. (Japan), Mitsubishi (Japan), Osaka Gas Co. (Japan), Allied-Signal Inc. (USA), Siemens AG (Germany), and International Fuel Cell Corp-(USA) etc.
Enthusiasm, and attention to this problem are generated by potential physical and chemical characteristics inherent in solid electrolytes and, therefore may be realized in electrochemical devices.
Currently, the HTFC based on zirconium dioxide in the tubular embodiment is theoretically the closest to an industrial realization. Currently, research and experimentation is being performed on the modified barium (strontium) cerate HTFC within the promising context of the proton electrolyte. This is confirmed by an analysis of the state of the art including scientific works dedicated to the proton electrolyte, and its wide discussion at international scientific seminars and conferences. On certain samples of thin-film (σ=10-15 μm), proton electrolytes with a current density of 0.3 A per cub. cm at 0.4 V and 600° C. was achieved. These values are about three times higher than those obtained with a zirconium dioxide based electrolyte under the same conditions, which is in fact explained by the extremely low polarizability of the conventional electrodes in contact with these ceramics and at a lower activation energy than that of the zirconium electrolyte at 800° C.
However, it is necessary to note that realization of the above-mentioned advantages of the electrochemical systems on a high level seems to be possible only when solving a problem common for all options: the problem of mastering the latest technologies, both in manufacturing the necessary ceramic materials, and in utilizing these materials during the process of manufacturing the HTFC itself. The most labor intensive problem is to create a technology of applying gas-dense layers (σ is equal of 1 to 40 μm) of electrolytes on the carrier (both gas-dense and porous). The thin-film, design has a particular significance for the construction of the proton electrolytes, because in this case the main disadvantages come from resistive loss, and the substantially lower mechanical strength of the BaCeO3 ceramic electrolyte, as compared to that of ZrO2. Therefore this material cannot simultaneously perform the two functions of the carrier element construction material and the electrolyte.
The technical problems of manufacturing a thin-film electrolyte are complemented by the high cost of the manufacturing technology. Thus, even if all technical problems were solved, the technology would contemplate the maintenance of parameters in order to obtain reproducible characteristics, for example, to control the temperature to within an accuracy of 2 degrees, would require an unacceptably expensive and possibly unreliable technology.
Currently known methods for two-stage high-temperature synthesis of oxide powder materials for manufacturing the thin-film electrolytes have been found to be unacceptable. Ultra-disperse powders which are nearly uniform by chemical composition and are used in forming electrolyte layers from 1 to 40 μm in width, require technologies without an intermediate step of powder preparation.
Methods without the steps of manufacturing and caking the solid electrolyte powders are known in the art. For example, in EVD technology used by Westinghouse, the step of synthesizing the solid electrolyte on the porous surface of the carrier cathode is performed from the decomposition of gaseous reagents, such as halogenides of zirconium and yttrium, onto the cathode. An advantage of this method is that layer forming may initialize with the participation of gas-phase molecular free particles. This solves the gas density problem. However, the necessity to use unique equipment and the necessity to work with caustic gas media such as halogenide compounds makes such technology unprofitable (U.S. Pat. No. 5,085,742, Int. Cl. H01M 6/00, H01M 8/00, published 1992). Another disadvantage of the CVD (EVD) method is the natural disproportion of the mixture of gas phases of the zirconium tetrachloride and yttrium trichloride during the process of their application onto the substrate. As a result, the yttrium within the applied electrolyte is non-uniformly distributed, and has a concentration gradient from the boundary within the substrate to the surface of the layer. Moreover, the cubic structure of zirconium dioxide (its monoclinic phase) appears not to allow the manufacture of the gas-dense electrolyte layer, with resistant characteristics, during the operation.
Another method for manufacturing components known in the art employs the organic compounds of elements such as Zr, Y, etc., for forming the electrolyte and organic compounds of elements such as La, Mn, Sr, etc., for forming the cathode, current passage, and anode etc. These compounds are easily disassociated under heating, thereby allowing the forming of HTFC components at relatively low temperatures—below 600° C., and can be utilized in inert media or oxygen-containing air, under standard atmospheric pressure without intermediate steps of applying the porous electrolyte layer, for example, by plasma spraying.
It follows from the analysis of publications and patent literature that methods for applying metal oxides from their organic compounds which were previously used only for obtaining the protective coatings over construction materials are being developed currently for tile solid oxide electrolytes and electrodes (Kuntagai Tozhija, Johota Hvozhi, Shindon Juji, Kondo Wakicki, Muzita Suzumu, Dyanki naganu oyobl kogyo butsuri kaganu; Anform water. Energy Theory Life. 1987, 55, No.3, pp. 269-270 [Obtaining the thin oxide films of perovskite type by pyrolysis of organic acids salts]; M. V. Perfiliev, A. K. Demin, B. L. Kuzin, A. S. Lipilin, “High-temperature gas electrolysis”, Moscow, Nauka, 1988, p.98; GB patent No 136198, C1. C1A, 1974; Japanese patent application No. 62-235475, 1987, Int. Cl. C23C 20/08, C03C 17/25) [Collection of scientific materials edited by acad. Spitsin V.1. “Composition, features and application of β-diketonates of metals”, Moscow, Nauka, 1978, pp.116-119][1]; Katrin NordVarhaug, Chun-hua Chen, Frik M. Kelder, Frans P. E. van Berkil and Joop Schoonman, “Thin Film Techniques for Solid Electrolyte Composites” European solid oxide fuel cell forum. May 6-10, 1996. Oslo/Norway. pg. 331-340 [2]; European patent No 0478185, Int. Cl. 6 H01M 8/12, HO1 M 4/86, published in 1991).
In the art of manufacturing electrolytes, a process for forming oxide film coatings during a thermal decomposition of acetylacetonates of, for example, Zr, Ce etc. is highly desirable. In so doing, forming oxide compounds having cubic structures due to the carbon stabilization is preferable. The cubic structure of the carbon-stabilized zirconium dioxide is stable in atmospheric air at temperatures up to 900° C. When the temperature exceeds this point the cubic form is transformed to a monoclinic one, due to the carbon's partial oxidation to CO and CO2. In derivatographic analysis, the authors of [1] were able to obtain a fully stabilized zirconium dioxide (CSZ) by way of joint dissociation of β-diketonates of the zirconium and cerium (with pivaloil-trifluorine-acetonate).
With respect to the method for manufacturing the electrolyte by technical essence and the result achieved in service, the method for manufacturing the Zr/Y film electrolyte by electrostatic application of a gas-drop emulsion of β-diketonates (ESD) to a medium, is the most similar to the claimed group of the inventions. The process is performed using acetylacetonates of the zirconium and yttrium (Zr(O2C5H7)4, Alfa/Y(O2C5H7)3, Alfa) [2]. The essence of the method is in spraying the gas-drop emulsion of β-diketonates mixture in a closed chamber. The substrate placed in the chamber is electrostatically charged up to 8-10 kV, and is heated to temperatures ranging from 250 to 430° C. As a result, the emulsion drops fall on the substrate and are thermally decomposed, creating the YSZ film on the surface.
In utilizing the ESD method it is necessary to use metal helates, namely acetylacetonates, having a high melting-point of 194° C. Therefore, it is necessary to dissolve acetylacetonates in ethanol, butyl-carbitol or other solvents, which do not allow a higher than 0.05 mole per cub. dm. concentration of zirconium dioxide in solution. Such low concentration limits the rate of film application to the level of 2 μm per hour. Moreover, since helate ligands are strongly coupled with the metal atom, the decay of zirconium acetylacetonate occurs via an intermediate product having a polymeric structure. This product is destroyed not with the segregation of, but with the destruction of whole helate ligands, and residues of their carbon chains compete in the crystal lattice of zirconium dioxide with yttrium oxide. Thus, with the Y2O3 removed, the zirconium dioxide is partially stabilized by residues of the carbon chains which are released from the crystal lattice of the zirconium dioxide in the form of CO and CO2 during further burning, and this results in forming a certain amount (3-4%) of a monoclinic structure in addition to the inevitable forming of pores.
As it was mentioned above, the current passage is one of the main HTFC components. Among the list of the requirements and characteristics that a current passage used in an HTFC should meet, the main requirement is a high electrical conductivity both in oxidizing and restorative atmospheres. Such characteristics may be achieved preferably by current passages manufactured to include noble metals or noble-metal based alloys. Such as the current passage disclosed in U.S. Pat. No 3,457,052 (Int. Cl. B21b, B21c, published in 1969). Considering the high cost of these materials, wide scale manufacturing of this type of current passages is practically impossible.
The current passages made of electric conductive materials on the basis of metal oxides and their compositions are more promising. These compositions include doped lanthanum chromite (LaCrO3), since it is sufficiently resistant both in an oxidizing medium or oxygen-containing space and in a restorative medium of the fuel gas.
One of the requirements placed upon a material used in a current passage made of LaCrO3 and used in SOFCs, is its gas-density. This will help to avoid any cross-leakage of fuel and oxidizing gases through the current passage. It is known, that it is difficult to manufacture the LaCrO3 with high density in conditions of high oxygen activity. In so doing, temperatures above 1600° C. are necessary. Such high caking temperatures become unacceptable when heating the LaCrO3 together with other SOFC components. But introducing a fusible substance to improve the caking temperatures in the form of a second phase with a significantly lower melting point (about 1400° C.), makes the LaCrO3 more dense in oxidizing media. However, this method for decreasing the caking point is unacceptable, since it produces loss of liquid-phase fusible eutectics into other components of the fuel cell causing substance and morphology changes leading to the loss of fuel cell's functional characteristics.
With respect to the method for manufacture of a current passage by the technical essence and the result achieved in service, the method including steps of synthesizing a powder of electron-conducting material based on doped lanthanum chromite, and further, thermally spraying the commutation layer of this material onto the unmasked sector of the air electrode is the most similar to the claimed group of the inventions (U.S. Pat. No. 5,085,742, Int. Cl. H01M 6/00, H01M 8/00, published in 1992).
The current passages manufactured according to that technology have lost a number of known inherent disadvantages for above mentioned reasons. Along with that, complicated technological equipment leading to a substantial rise in the cost of the whole article is necessary in order to manufacture a current passage of sufficient gas-density from lanthanum chromite by the given method.
The main physical and chemical processes take place on the interface surfaces of the high-temperature electrochemical devices, enabling the operation of the device as a whole.
A chemical composition of single HTFC layers including a positive electrode, an electrolyte, and a negative electrode (PEN), is usually selected according to maximum electric (electronic, ionic) conductivity. In general, this selection rarely coincides with other functionally necessary requirements to the materials, for example, chemical and thermal stability, constructional strength etc.
On the interface surfaces between the electrodes and the electrolyte, chemical interactions are possible in working temperature conditions or during the process of manufacturing. Thus, on the LSM/YSZ boundary, a formation of the lanthanum zirconate La2Zr2O7 occurrs, leading to a sharp increase in the contact resistance and to a deterioration of cathode operation. ([Andreas Mitterdorfer et al. ETN Zurich “Department of materials, Swiss Technology Institute”], Second ESOFC Forum Oslo (Norway), May 1996, pp. 373-382).
A promising use of the modified cerium oxide having an ionic conductivity higher than that of the modified zirconium dioxide, is as the electrolyte. However, in the restorative fuel medium, the cerium oxide has noticeable electronic conductivity, which may lead to the decrease of the EMF (voltage of an interrupted circuit of PEN) by 20-30% (M. Sahibzada et al., Department of materials and department of chemical engineering technology, Imperial college of science, technology and medicine, London, Second ESOFC Forum Oslo (Norway), May 1996, pp. 687-696).
Currently, manganite doped with strontium or lanthanum is one of the most promising materials for an air electrode. However, since it is easily restored with a sharp increasing of the TEC from 12.5*10−6 to 14.5*10−6 K−1 even in a slightly restorative medium, its application is possible only with absolutely dense layers of the solid electrolyte. Any local decreased density in the electrolyte being in contact with MLS leads rapidly to the destruction of the whole element. Since current techniques display a tendency to decrease the electrolyte thickness (to 5 μm and below), there is a great probability for the existence of local microdefects.
In order to solve the problem of effective service material mating, and also of increasing material stability when in gas media operation, it is necessary to form special intermediate layers, or interface layers, on the interface surfaces. As a rule, the thickness of interface layers doesn't exceed units of micrometers.
The creation of such thin interface layers imposes special requirements on the technology used to form them. Until 1994, two processes could be considered the most acceptable: an electrochemical application from the vapor-gas phase (EGP), and a magnetron spraying (both are derived from the technology for epitaxial layer growth in the integrated circuit industry). Both techniques are economically unprofitable in HTECD production due to the expensive equipment and high operation expenses necessary for their practical implementation.
Among the cheaper technological processes, a pyrolysis dissociation of metal-organic complexes, metal-organic compounds, or their mixtures is the most promising. An electrostatic pyrolysis spraying (EPS) is one variation, being the most similar to the present group of the invention with respect to manufacturing the interface layer [2].
The second electrode of HTFC is the anode, which, as a rule, is manufactured from the cermet.
On the basis of the above-mentioned requirements imposed on electrodes and caused both by the technologies for their manufacturing, and by operating conditions, it can be established that the most significant among these requirements is the selection of the material used for the anode manufacturing. In so doing the following circumstances are taken into account:
Because of the restorative conditions in the fuel gas atmosphere, metals are used as an anode material for SOFC. Since the material composition may change during the operation of the fuel cell, the used metals should not be oxidized both in pure fuel conditions and in maximum oxidation conditions of the fuel during output from the fuel device.
Under operation temperatures in the range of 700° to 100° C. in the SOFC with a solid electrolyte, the list of used metals is limited, in general, by the nickel, cobalt and noble metals. The nickel is the most common due to its low cost (as compared to cobalt, platinum etc.).
In order to obtain the cermet anode with a porous structure operating over a long period of time at 700-1000° C., the nickel in the form of metal is usually used together with the stabilized zirconium dioxide and also with the stabilized cerium oxide, which is necessary for additional (internal) reforming of the fuel gas.
A substrate of an ion-conductive ceramic material keeps metallic nickel particles, and prevents the caking of metal particles at the operation temperatures of the fuel cell.
When manufacturing the anode cermet for the SOFC, it is usual to begin with powder materials such as YSZ and NiO. The mixture is may be formed into a compact electrode by different methods. Further, the NiO will be restored to the nickel metal under the effect of the fuel cell. For thin layers (for example, 100 μm in thickness) of the nickel cermet annealed in air, several minutes are necessary to complete the restoration process for the NiO at temperatures around 1000° C.
The method for manufacturing the cermet fuel electrode is the most similar to the present invention in the manufacturing aspect of the cermet fuel electrode. The method comprises steps of:
applying a separate electron-conductive layer onto the external porous electrode coupled to the solid ion-conductive electrolyte which, in its turn, is in contact with the internal electrode,
injecting into the external porous electrode a mixture consisting mainly of a salt containing a first metal, the metal containing component being chosen from the group including nickel, cobalt, and their mixture, and the salt being chosen from the group including nitrate, acetate, propyonate, butyrate, and their mixture, or from their mixture mainly consisting of a salt containing a second metal, the metal component being chosen from the group containing cerium, strontium titanate, and their mixture, and the salt being chosen from the group containing nitrate, acetate, propyonate, butyrate, their mixture, and also a non-ionized surface-active substance.
heating the applied mixture in the atmosphere up to a sufficient temperature for forming a separate solid porous electron-conductive, multi-phase layer mainly consisting of a conductive oxide chosen from the group containing cerium oxide, strontium titanate, and their mixture. The thin metal particles contained in it being preferably chosen from the group consisting of separate nickel particles, separate cobalt particles, and their mixture; the particles being from 0.05 to 1.75 μm in diameter.
In addition, an external porous electrode contains large metal particles from 3 to 35 μm in diameter. The electrode is partially introduced into the structure of the substrate including the stabilized zirconium dioxide portion, and the thin metal particles which range from 0.25 to 0.75 μm in diameter.
In addition, dopants used with the second metal contained in the salt are selected from the group, including Mg, Sr, Ba, La, Cc, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, Y, Al, and their mixtures. A method for their heating, performed at the rate of 50 to 100° C. per hour, is disclosed in U.S. Pat. No. 5,021,304 (Int. Cl. H01 M 8/10, published in 1991).
A disadvantage of the this method for anode manufacturing is its multiple stages and large number and great variety of classes of utilized organic reagents. The proposed invention provides for a fuel electrode using a single physical/chemical process during a single operation with a single class of organic reagents, namely, a mixture of dimethyl-carbonic acids, where the straight carbon chain can be represented as a row from C1 to C12. Such combination of carbonic acids is the cheapest and the most wide-spread in the industrial-scale production of organic reagents.
An electrical insulating layer is a necessary element of the HTFC construction. In order to operate the HTFC it is necessary to provide electrical insulation between the electrolyte, anode and current passage. The electrical insulating material in the HTFC construction contacts the electrolyte and the anode and cathode materials. In this connection, the most strict requirements are imposed on contacts of the electrically insulating material, which relies on the fact that such contacts must maintain certain mechanical and gas-density requirements under operating conditions and must exclude the interaction of the materials which may lead to the loss of HTFC serviceability.
The most common requirements placed on the electrically insulating material are the stability of its structure, high temperature material characteristics, reliable contact with interfaced materials, an absence of the interaction in the contact zone, and a TEC value in proximity to the value of the interfaced material TEC.
Ceramic materials based on the magnesium spinel Al2O3 and/or MgO have acceptable insulating features (DE Patent No. 2756172. Int. Cl. 6, C25B 9/04 published in 1979).
Solid electrolytes based on zirconium dioxide are characterized by a sufficiently low electric conductivity with acceptable levels of the stabilizing additives and chemical compounds of zirconium; for example, zirconates, which determine the possibility of their usage as electrically insulating materials. In addition, their TEC values coincide well with the TEC value of the solid zirconium electrolytes.
Under working temperatures conditions for HTFC operation, the doped magnesium spinels possess many desirable characteristics, including good electrical insulating characteristics. As a rule, doping is performed in order to bring the TEC of the HTFC component materials interfaced (the electrolyte, electrode, and current passage) with that of the electric insulator. The final step in the manufacturing technology of an HTFC cell is the step of applying the electrically insulating layer. After which, the cell is ready to be placed into a stack, i.e., it does not seem possible to exceed the temperatures of forming the previous HTFC layers (components). At the same time, the caking temperatures of the electrical insulator materials are relatively high. The application of additives for obtaining fusible eutectics is unacceptable in this situation because of the ease at which they diffuse into the HTFC components which are interfaced with the electrical insulator, thereby disrupting their functional characteristics.
The method disclosed in (DE Patent No 2756172 Int. Cl. 6, C25B 9/04, published in 1979) for manufacturing an electrically insulating layer, comprising the steps of preparing the component compound on the basis of a magnesium spinel using fusible eutectics, is the most similar to the present group of inventions with respect to preparing the electrically insulating layer in its technical essence and the result achieved in service.
A typical sequence of arranging and manufacturing the HTFC components may include the following:
a porous carrier cathode;
a layer of electrode material of no more than 0.6 μm in thickness, contributing the activation of electrode processes, applied to the porous substrate of the carrier electrode material;
a current passage;
a gas-dense layer of the Ce(Sm/Gd)O2-x, of 5 to 10 μm in thickness on the surface of the active electrode material contacting the MLS, wherein said layer is working as an electrolyte;
gas-dense layer of the YSZ on the surface of the Ce(Sm/Gd)O2-x of 3 to 5 μm in thickness for preventing the restoration of the doped cerium oxide (the electrolyte) by the fuel gases;
a fuel electrode cermet; and
an electrical insulating layer.